Ecological Applications, 6(1),, 1996, pp. 326-334
0 1996 by the Ecological Society of America
MICROHABITATS OF MERRIAM’S TURKEYS IN THE
BLACK HILLS, SOUTH DAKOTA1
M ARK A. RU M B L E
USDA, Forest Service, Rocky Mountain Forest and Range Experiment Station, 501 E. St. Joe,
Rapid City, South Dakota 57701 USA
S TANLEY H. AN D E R S O N
USDI, Fish and Wildlife Service, Cooperative Fish and Wildlife Research Unit, University of Wyoming,
Laramie, Wyoming 82071 USA
Abstract. Merriam’s Turkeys (Meleagris gallopavo merriami) are associated with ponderosa pine (Pinus ponderosa) forests in the western United States, but are not native to
the ponderosa pine forest of the Black Hills, South Dakota. The Black Hills population
was established by transplanting birds from New Mexico and Colorado between 1948 and
195 1. Despite being outside its original range, this population provides a unique opportunity
to assess mechanisms of habitat selection because the age of the population is known and
literature indicates that it is more productive than other populations. We studied microhabitats of Merriam’s Turkeys in the Black Hills, South Dakota between 1986 and 1991.
We found few differences in microhabitats among diurnal time periods or between sexes.
Cluster analysis of variables at turkey microhabitats indicated two groups, broadly interpreted as summer and winter microhabitats. Winter microhabitats of turkeys had less understory vegetation and more overstory cover than random sites, which in turn had less
understory and more overstory cover than summer microhabitats. Both random sites and
winter microhabitats had higher basal area of ponderosa pine than summer microhabitats.
Summer microhabitats had trees with the largest dbh. Random sites had more small and
large woody debris than sites used by turkeys. Tree density at random sites was more than
two times greater than at winter microhabitats and more than three times greater than at
summer microhabitats. Turkeys preferred southern exposures during winter. Production of
pine seed, a major food item of turkeys, differed among years. There was a strong relationship between abundance of pine seeds and microhabitats selected by turkeys. Basal
area of microhabitats between October and March was positively correlated with annual
ponderosa pine seed production. Abundance of ponderosa pine seeds at turkey microhabitats
during this period was at least four times the estimated average annual production. Management prescriptions for ponderosa pine of basal area 5 18 m2/ha will reduce winter habitat
for turkeys. Summer habitats are more compatible with timber management goals for ponderosa pine in the Black Hills.
Key words: food resources; forest management; habitat selection; Meleagris gallopavo merriami;
Merriam 's Turkeys; microhabitat relationships.
Ponderosa pine (Pinus ponderosa) is associated with
habitat of Merriam’s Turkey (Meleagris gallopavo merriami) throughout the species’ range (MacDonald and
Jantzen 1967). Merriam’s Turkeys were transplanted to
South Dakota between 1948 and 195 1 (Petersen and
Richardson 1973). Despite being outside of the subspecies’ original range (MacDonald and Jantzen 1967),
turkeys in the Black Hills are more productive (Rumble
and Hodorff 1993) and sustain higher annual harvests
than other populations of Merriam’s Turkey (Kennamer
et al. 1992). These population characteristics, when
linked to habitat selection and vegetation patterns, provide a unique opportunity to examine habitat selection
and explore its mechanisms in Merriam’s Turkey.
1Manuscript received 6 May 1994; revised 3 January 1995;
accepted 12 January 1995; final version received 26 January
1995.
During the past 10 years, commercial cutting of ponderosa pine in the Black Hills has increased 55% and
the sale value has increased more than eight-fold (unpublished manuscripts: Land and Resource Management Plan 1983, Draft Land and Resource Management
Plan 1994, Black Hills National Forest, Custer, South
Dakota). This level of cutting presents unknown impacts on South Dakota’s largest turkey population. Currently, 11 forest management plans identify Merriam’s
Turkey as a Management Indicator Species. Yet, data
for most elements of Merriam’s Turkey ecology are
lacking to develop management standards and guidelines.
Early studies of habitat use and management of Merriam’s Turkey outside the original range (Jonas 1966,
Petersen and Richardson 1973) were biased because
they depended on visual observations and, consequently, may have overemphasized open habitats where birds
326
February 1996
l
*
MERRIAM’S TURKEY MICROHABITATS
were more visible. Recent studies have focused on
unique aspects of Merriam’s Turkey habitat selection
by use, such as roosting (Mackey 1984, Lutz and Crawford 1987a, Rumble 1992), brood-rearing (Mackey
1986, Day et al. 1991a, Rumble and Anderson 1993a),
and nesting (Lutz and Crawford 1987b, Day et al.
1991b, Rumble and Hodorff 1993).
Macrohabitats (habitats delineated from dominant
vegetation types or land units ranging from 4 to 32 ha
in size) of Merriam’s Turkey have been studied (Lutz
and Crawford 1989, Rumble and Anderson 1993b).
Broad effects of land management activities, such as
logging, can be hypothesized at the macrohabitat scale
if habitats are stratified properly (Rumble and Anderson 1992). However, understanding the character and
magnitude of impacts from timber harvest and mechanisms of habitat selection requires precise knowledge
of how birds use forested habitats. Microhabitats, or
fourth-order habitat selection (Johnson 1980), provide
this level of resolution. To our knowledge, no previous
studies have evaluated microhabitats of Merriam’s Turkey in ponderosa pine forests.
We studied microhabitat selection patterns of Merriam’s Turkeys in a ponderosa pine ecosystem and evaluated the relationships between microhabitat selection
and food resources. We tested hypotheses that (1) microhabitat of Merriam's Turkeys did not differ among
daily time periods, between sexes, or among seasons;
(2) microhabitat did not differ between available conditions and conditions at sites used by Merriam’s Turkeys; and (3) microhabitat selection by Merriam’s Turkeys was not attributable to the distribution and abundance of food resources.
S TUDY A REA
4
4
AND
METHODS
We studied Merriam’s Turkey habitats on 4380 ha in
the central Black Hills of South Dakota from February
1986 to December 1991. Geologic material is predominantly Precambrian and Cambrian granite, schists, and
metasediments (Hoffman and Alexander 1987). Climate is continental, with cold winters and warm summers (Orr 1959). Temperature extremes range from
-34 to 3 8 o C and precipitation averages 50-55 cm
(South Dakota Climatological Summary, Number 2039-6, U.S. Weather Bureau). Climate and soils in the
Black Hills are ideally suited for ponderosa pine (Boldt
and Van Duesen 1974), which comprises 84% of the
area. Rumble and Anderson (1993a) provide detailed
descriptions of the study area vegetation.
Some turkeys in the Black Hills use ranch feedlots
and/or suburban housing developments for winter feeding; others remain in the forest throughout the winter
unless deep snow forces them to use these unnatural
food sources. Our research focused on a population in
natural forest.
We trapped Merriam’s Turkeys during late February
or early March with alpha-chloralose (Williams 1966),
rocket nets, and drop nets. Radio transmitters were at-
327
tached with 0.6 or 0.9-cm bungee cord looped under
the wings. We radio-marked 111 turkeys, 80 females
and 33 males; most males were radio-marked after
1988.
We began locating turkeys 1 wk after attaching the
radio transmitters (Nenno and Healy 1979). We attempted to locate each bird three times a week, once
each during morning (sunrise-l 000), midday ( 10011400), and afternoon (140 1 to sunset). One location
each week was a precise location determined by direct
observation or close-range telemetry with a hand-held
two-element yagi antenna. Close-range telemetry locations were later confirmed by droppings or scatchings
by wild turkeys.
Microhabitat descriptions
Vegetation characteristics (microhabitats) were sampled at each precise location, usually 5 1 wk following
the location date. We sampled microhabitats along a
60-m transect centered over the bird’s location and oriented along the contour. This transect was constrained
to be within the same vegetation type in which the birds
were observed. At the midpoint of the transect, we
estimated percent slope using a clinometer and aspect
as the downhill compass bearing. At the beginning,
midpoint, and end of the transect, we used a 10-factor
prism (Sharpe et al. 1976) to estimate tree basal area
(BA) and to determine trees from which to measure
diameter at breast height (dbh). Tree density was calculated from these data. We also estimated percent
overstory canopy cover using a spherical densiometer
(Griffing 1985) at these three points along the transect.
Data from the beginning, midpoint, and end were averaged for estimates of variables at each microhabitat.
We estimated percent canopy cover (Daubenmire
1959) of total vegetative cover, grasses, forbs, shrubs,
and shrub species at l-m intervals along the transect.
Following abscission of shrubs, coverage was estimated from the outer extent of woody material. Following senescence of grasses and forbs, we estimated
coverage of these categories from plant material of the
previous growing season. Numbers of shrub species
and soft mast-producing shrubs were tallied from these
data.
Beginning in January 1989, additional measurements
were collected at turkey microhabitats. We estimated
the quantity of downed woody debris at each site for
small (2.5-7.5 cm) and large (>7.5 cm) diameter logs
(Photo series for quantifying forest residues in the
Black Hills, USDA Forest Service, Rocky Mountain
Region, AFM 831, 1982). Timber management prescription was classified as: not cut (no evidence of past
timber management); clear-cut (most or all trees removed in an area pattern); commercially thinned (evidence that past cutting had removed several-to-most
mature trees, including selective cutting); shelterwoodseedtree (most trees harvested, with remaining trees of
mature, seed-producing size, BA 5 9 m2/ha); and pre-
328
MARK A. RUMBLE AND STANLEY H. ANDERSON
commercially thinned (small diameter trees cut and left
in the forest). Time since cutting was estimated as <2
yr, 2-5 yr, and >5 yr. We also subjectively categorized
each site as having a single- or multilayered canopy.
Multilayered stands had one or more age classes of
trees beneath the canopy of the tallest tree class.
Beginning in January 1989, we estimated the abundance of ponderosa pine seeds at each turkey microhabitat sampled between October and April. Litter was
collected from three 0. 1-m2 quadrats at the beginning,
midpoint, and end of the transect and the pine seeds
were counted.
Twelve macrohabitat categories based on dominant
vegetation type, dbh, and overstory canopy cover (Buttery and Gillam 1983) were identified in our study area.
These included meadows, three overstory canopy cover
categories of aspen/birch (Populus tremuloides/Betula
papyrifera) , two dbh categories of ponderosa pine with
three overstory canopy cover categories in each (six
total), bur oak (Quercus macrocarpa) ,and white spruce
(Picea glauca) . Using forest inventory maps from the
Black Hills National Forest and interpreting aerial photographs, we delineated 5 12 macrohabitat land units (4
to 32 ha in size) in our study area. Each fall, from 1987
to 1991, we placed three 0.5-m2 circular plots with
nylon screen in three replicates of each of the six ponderosa pine macrohabitat categories to estimate annual
pine seed production. We placed the plots in the field
during the 1st wk of September and removed them the
last week of October. Because these plots were not
animal-proof, we counted seed wings with bracts indicative of a mature seed as an estimate of seed production.
We sampled 572 random sites using the same procedures as at turkey microhabitats between 1987 and
1991. Random sites were selected from a stratified random sampling of the macrohabitats described above.
Ten sites in each of 11 macrohabitat categories and five
sites in bur oak were marked on 1:24,000 US Geological Survey contour maps in the laboratory and relocated in the field each year. Incomplete data caused us
to eliminate three sites.
Analyses
Microhabitat variables of trees and shrubs with < %
frequency of occurrence at turkey sites or comprising
< 1% canopy cover were excluded from analyses. Data
from uncommon species are highly variable and often
cannot be interpreted (Uresk 1990).
We tested for homogeneity of variance and normality
in our data and these assumptions were seldom met.
When possible, the procedures used for analyses did
not make these assumptions of parametric statistics, or
included adjustments for violations. We tested hypotheses that microhabitat variables did not differ among
the three daily time periods with Welch’s test (Milliken
and Johnson 1984), and used Dunnett’s T3 procedure
for heterogeneous variances (Dunnett 1980) to make
Ecological Applications
Vol. 6, No. 1
multiple range comparisons. For data collected after
1988, we tested hypotheses that microhabitat variables
at sites selected by male and female turkeys did not
differ, using separate variance estimate t tests.
We then used an iterative cluster procedure based on
Euclidean distance (del Moral 1975) to explore patterns
of microhabitat selection by Merriam’s Turkeys. Inclusion of trivial variables in cluster analysis can produce
spurious clusters (Everitt 1977:69). Therefore, we used
principal component analysis (PCA; Everitt 1977) to
reduce our data to nine variables that captured the majority of the variation in the microhabitat data. These
variables included percent total vegetative cover, percent forb cover, percent grass cover, percent shrub cover, percent kinnikinnick (A rctostaphylos uva-ursi) cover, number of shrub species, percent overstory canopy
cover, ponderosa pine BA, and tree density. To ensure
that all variables received equal weight in the cluster
analysis, variables were standardized to unit standard
deviations.
We then conducted analyses to compare microhabitat
characteristics among sites grouped by the cluster analysis and random sites. These analyses are not confirmatory of the cluster analysis, but evaluate variables important to the cluster solution and patterns relative to
random availability. A priori review of the literature
before conducting the cluster analysis suggested many
variables were important in Merriam’s Turkey habitat
selection (Scott and Boeker 1973, 1975, 1977; Habitat
Suitability Index Models: Merriam’s Turkey, USDI
Fish and Wildlife Service, Fort Collins, Colorado, unpublished manuscript) .Therefore, we considered all
variables that met the initial 5% frequency and 1%
cover criteria (prior to PCA) and variables added after
January 1977 in these analyses (Tables 1 and 2). We
used Welch’s test and Dunnett’s T3 multiple range procedure to test hypotheses that microhabitat variables at
random sites did not differ from turkey sites. Random
data were weighted to account for deviations from proportional sampling. Values of each variable at each site
were multiplied by pi572/ni, where pi represents the
proportion of the area in the ith macrohabitat category,
and ni represents the number of random samples in the
ith macrohabitat .
We categorized aspect as north (316-450 o ), east (46135 o ), south (136-225 o ), and west (226-315 o ). Chisquare goodness-of-fit was used to test hypotheses that
random sites and sites used by turkeys had the same
aspect (Batschelet 1965), timber management prescription, time since harvest, and stand structure. We used
confidence intervals with a Bonferroni correction to
preserve the experimentwise error (Neu et al. 1974)
around proportional use of groups from cluster analysis
to determine categories at turkey microhabitats that differed from random.
We tested hypotheses that ponderosa pine seed production did not differ among years using multiresponse
permutation procedure (Mielke 1984), and used sepa-
February 1996
MERRIAM’S TURKEY MICROHABITATS
331
Average ponderosa pine seed availability (seeds/m*) beneath the ponderosa pine stands and October-March turkey
microhabitats in the Black Hills, South Dakota, 1987-1991.*
T ABLE 3 .
1987-1989
x
Ponderosa pine stands
Turkey microhabitats
c
30.1
NA
SE
A
3.6
1988-1989
x
SE
2.0 c 0.3
15.2 AB 3.3
1989-1990
x
3.2 BC
11.5 A
SE
0.5
1.6
1990-1991
1991-1992**
x
x
SE
2.8 BC 0.5
18.6 AB 1.9
SE
5.3 B 0.7
24.0 B 10.7
* Means with different letters across rows differ at a 5 0.05, MRPP (multiple random permutation procedure).
** The study was completed in December 199 1.
and Hoffman 1993), but habitats of both sexes are similar (Cade and Hoffman 1990).
Seasonal patterns of habitat selection
Merriam’s Turkeys in the Black Hills used two
groups of microhabitats that differed in vegetation and
represented microhabitats typically selected during
summer and winter. Turkeys selected both groups nearly equally during the spring and to a lesser extent in
fall. Patterns of macrohabitat selection (land units 432 ha) also were less apparent during spring and fall
(Rumble and Anderson 1993b). The spring and fall
seasons in this region seldom blend gradually from
winter to summer or vice versa. Weather conditions
typical of both summer and winter commonly occur
during spring. Snow storms with accumulations up to
30 cm are often followed by temperatures above 16oC,
but with decreasing frequency, during March and April.
Patterns of microhabitat selection by turkeys reflected
these conditions. During fall, microhabitat selection
patterns changed rapidly; by November, microhabitats
typical of winter were predominately selected.
Winter.-Winter microhabitats of Merriam’s Turkeys in the Black Hills were characterized by ponderosa pine with little or no understory vegetation, high
BA, and high overstory canopy cover. Selection of macrohabitats during winter reflected similar patterns
(Rumble and Anderson 1993b). An open understory is
characteristic of winter turkey habitats in the southeastern United States (Exum et al. 1987).
Ponderosa pine seed, when available, is the primary
winter food item of Merriam’s Turkeys (Jonas 1966,
Scott and Boeker 1973, Rumble 1990). The turkeys’
selection of microhabitats reflected their search for pine
seed. Similarly, Capercaillie (Tetrao urogallus) habitat
selection is determined by the abundance of billberry
(Vaccinium myrtillus) (Storch 1993). Turkeys foraged
for pine seed in stands with high BA and little understory vegetation. Pine seed abundance increases asymptotically with BA of ponderosa pine up to -27 m2/
ha; during years of low pine seed production, this relationship is not evident (Rumble 1990). BA and understory vegetation are inversely related (Uresk and
Severson 1989), which could account for the low understory vegetation at winter turkey microhabitats.
Birds would find it more difficult to search for pine
seeds in pine stands with low BA because of the greater
amounts of understory vegetation. Also, more pine
seeds occur in stands with high BA. Average BA of
microhabitats selected from October to March corresponded with annual production of ponderosa pine
seed.
Drought and high temperatures between May and
August 1988 (South Dakota Climatological Summary,
U.S. Department of Commerce, Asheville, North Carolina) were followed by decreased production of pine
seeds in the fall. Pine seed was uncommon in turkey
diets after October 1988; kinnikinnick seeds and grass
leaves and seeds comprised the majority of turkey diets
throughout the winter (Rumble 1990), and birds selected microhabitats with BA more typical of summer
microhabitats. We observed turkeys pecking at cones
from previous years on several occasions and, despite
a mild winter, 25% of our radio-marked birds, all subadult females, succumbed to predators 1 wk following
a spring storm with 31 cm of snow. In Pennsylvania,
turkeys starved when snow exceeded 0.5 m for >2 wk
(Wunz and Hayden 1975). With the absence of adequate
nutrition, snowfall >25 cm resulted in increased mortality (Austin and DeGraff 1975, Porter et al. 1980)
and reduced reproductive performance (Porter et al.
1983) in Eastern Turkeys (M. g. silvestris). Effects of
severe winter weather were more pronounced on juveniles than adults (Porter et al. 1980).
Habitat selection is, in part, an expression of foraging
behavior and dietary need and, therefore, is predictable
using foraging theory (Rosensweig 1985). We cannot
draw conclusions as to whether or not these microhabitats maximized energy intake rate (Pyke 1984) for
birds. Munger (1984) concluded that selecting optimal
habitats may be less important than selecting habitats
that are better than random. Turkeys in our study selected winter microhabitats where pine seeds were
more abundant than random.
Merriam’s Turkeys in the Black Hills preferred winter microhabitats with southern aspects. Turkeys in
Oklahoma preferred south-facing habitats and avoided
north-facing habitats despite greater mast production
on them (Bidwell et al. 1989). We believe that turkeys
selected southern aspects because these sites were
warmer and snow melted sooner. South slopes were
used by turkeys in New York because they have less
snow for shorter periods (Austin and DeGraff 1975).
Feeding behavior and daily movements of turkeys were
adversely affected by snow depths > 15 cm (Austin and
DeGraff 1975, Good 1982), and birds were incapaci-
332
MARK A. RUMBLE AND STANLEY H. ANDERSON
tated by >30 cm of fresh snow (Austin and DeGraff
1975). During and immediately after snow storms, turkeys stayed in roosts or were inactive on the ground
throughout the day. Eastern Turkeys were inactive and
remained in roosts during the day periods when snow
was deep and there were inadequate food supplies (Porter et al. 1980).
Winter is a stress period for Merriam’s Turkeys in
the Black Hills. The average median daily temperature
between December and February during our study was
-4 o C. Given the average range of daily temperature
(14.2oC), ambient temperatures were often near or below the lower critical thermoneutral temperature (Tlc)
of Eastern turkeys during this period. Merriam’s Turkeys are similar in size to Eastern Turkeys (Stangel et
al. 1992), and would be expected to have a similar Tlc
of -15OC (Oberlag 1989).
Annual variation in production of the primary food
(ponderosa pine seed) influenced the microhabitats selected. We attempted to study a turkey population that
did not depend on unnatural foods. However, birds used
trap bait, livestock feedlots, and bird feeders at private
residences following fresh snowfall of > 15 cm, regardless of annual pine seed production. During winters
of low pine seed availability, radio-marked birds used
unnatural food sources regularly. Deep snow prevents
turkeys from scratching for food (Austin and DeGraff
1975). Turkeys depend on agriculture fields or other
unnatural foods during periods of deep snow and the
absence of natural foods (Austin and DeGraff 1975,
Porter et al. 1980).
Summer. -Summer microhabitats of turkeys(excluding females with poults) included ponderosa pine
habitats with less BA than winter habitats and greater
understory cover of grasses, forbs, and shrubs. During
this period, turkeys primarily consumed seeds from
grasses and other vegetative materials (Rumble 1990).
For loafing and escape cover, birds chose microhabitats
with high BA, which partially accounts for selection
of microhabitats with winter characteristics during
summer. Merriam’s Turkeys in Montana used dense
sapling pine stands for loafing (Jonas 1966). Hoffman
et al. (1993) recommended stands with BA >23 m2/ha
as loafing cover for Merriam’s Turkeys.
M ANAGEMENT IM P L I C A T I O N S
Timber management goals for the Black Hills National Forest are to manage BA to 5 18 m2/ha (Land
and Resource Management Plan 1983, Black Hills National Forest, Custer, South Dakota). These timber
management goals are usually attained through commercial timber cutting or pre-commercial thinning.
Management that reduces BA to 5 18 m2/ha will reduce
the quality of winter microhabitats. If turkey populations that are not completely dependent on artificial
food sources during winter are a goal, management will
need to ensure adequate abundance and distribution of
winter habitats. Precise estimates of how much winter
Ecological Applications
Vol. 6, No. 1
habitat is required for sustainable turkey populations
are not available. However, Hoffman et al. (1993) recommended that 25% of ponderosa pine habitats should
have BA >23 m2/ha, 15% of which should have BA
>30 m2/ha. Some winter habitats should occur on
southern aspects (135-225o).
Summer microhabitats are mostly compatible with
timber management in the Black Hills. Seedtree cuts
typically have BA <9 m2/ha, less than birds selected
for at summer microhabitats, and are used to regenerate
stands (a necessary part of multiple use management).
Because seedtree harvest methods are not the only timber management prescription used in the Black Hills,
their prudent use should not impact summer microhabitats of turkeys.
ACKNOWLEDGMENTS
The USDA Forest Service, Rocky Mountain Forest and
Range Experiment Station and Black Hills National Forest,
National Wild Turkey Federation, and South Dakota Game,
Fish and Parks provided financial support for this research.
Dr. A. J. Bjugstad (deceased) provided initial advice and encouragement. R. A. Hodorff, T. R. Mills, C. D. Oswald, K.
J. Thorstenson, K. L. Jacobson, and L. J. Harris provided
technical assistance. M. P. Green was a volunteer throughout
this study, and R. L. Taylor allowed access to his property.
J. A. Crawford, H. G. Shaw, B. F. Wakeling, and two anonymous referees reviewed earlier drafts of this manuscript. The
procedures we used followed protocol established for use of
wild birds in research (American Ornithologists Union 1988).
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